In data communications systems, the data to be transported or communicated is transported over a channel, that may have unknown, indeterminate, or varying characteristics and the data is therefore subject to various forms of corruption. Such channel parameters include, for example, channel length or time delay, channel gain or more accurately loss, and various additive signals such as noise or DC (non time variant) offsets. Additional forms of corruption may be encountered because of the data communications equipment employed or other conditioning of the data required to facilitate transport of the data over a particular channel.
The latter issues may become particularly pronounced in wireless data communications systems. These systems typically employ some form of modulation of a radio frequency carrier, use channels that may be especially difficult to characterize, and employ equipment with inherent variations that, without more, could materially impact the integrity of the data as received. The net of all this is that practitioners have accepted the notion that a wireless data communications system and more specifically a receiver or demodulator will have to take affirmative steps to accurately demodulate or reproduce the data that was intended to be transported over the wireless channel.
It is now routine for data receivers or demodulators to provide, for example, some form of estimate of the received signal's amplitude, DC offset, and phase or timing. These estimates are in turn used to enhance the demodulator's overall accuracy by removing or minimizing disadvantageous effects resulting from, among others, the above sources of corruption. For example, a DC offset, resulting from a frequency difference between a transmitter and the receiver in a frequency modulation system, may be estimated by finding and tracking over time the average of the peak and the minimum received signal level and subtracting this average from the received signal.
As is well known, if this DC offset is accurately estimated and the received signal is adjusted by removing this estimate the data demodulator can more accurately distinguish between possible data symbols and therefore more accurately recover the data that was transported. Similar scenarios hold for the amplitude and phase or timing estimates. In any event the bit error rate (BER) performance of the data communications system will be improved or will suffer in accordance with the accuracy of these various estimates. Present data demodulators first provide these estimates, then adjust the received signal in accordance with these estimates, and finally select a data symbol for a symbol time in response to this adjusted receive signal. These demodulators operate satisfactorily on some forms of modulation but are systematically deficient for more spectrally efficient modulation approaches or any modulation approach that has a so called partial response.
Partial response is used to designate a characteristic of modulation approaches where the effect on a carrier of a particular symbol for a particular symbol time is dependent on the symbols that occurred in one or more preceding symbol times. For example, consider a RF carrier and a two level gaussian minimum shift keyed (GMSK) modulation approach. Under these circumstances the maximum frequency deviation of the RF carrier and thus maximum received signal amplitude may not be realized or observed until more than one identical symbol has been transmitted and hence received. Therefore estimates of the maximum amplitude made with or as the result of a sole symbol may be erroneous and the data communications system BER performance can suffer. Clearly a need exists for a data receiver and data decoder that employ methods and procedures to account for modulation approaches that have partial responses.